Digital offset printing presses use an offset cylinder to transfer a color image to a paper, film or metal substrate. These presses are based on an electrophotographic process where an electrostatic image is formed on a surface of a charged photo-conductor by exposing the surface to a light and shadow image to dissipate the charge on areas of the photo-conductor surface exposed to the light. A charged electrostatic latent image is then developed using ink (liquid toner) containing an opposite charge to that on the electrostatic latent image. The resulting electrostatic toner image is then transferred to a hot blanket, where the toner is coalesced and maintained in a tacky state until it is transferred to the substrate, which cools the ink and produces a tack-free print.
Conventional digital offset receiving sheets are typically highly cationic in nature, are not thermally stable at temperatures above 200° C., and commonly employ a substrate prepared using a polyamine or quaternary ammonium polymer.
Xerography is an electrophotographic technique that uses electrostatic charges, dry ink (toner) and a fusing process to produce and fix an image on a substrate. A negative of an image is beamed using a laser or light-emitting diodes (LEDs) onto a positively charged selenium-coated, photoconductive drum, thereby canceling the charge and leaving a positively charged copy of the original image. A negatively charged toner is attracted to the positive image on the drum. The toner is then attracted to the substrate (e.g., paper), which is also positively charged. Heat and/or pressure or light is then used to fuse the toner thereby causing it to permanently adhere to the substrate. Receiver sheets used in xerographic printers and presses utilizing heat and pressure fusing may be exposed to temperatures ranging from about 140° C. to about 250° C.
Conventional receiver sheets used in xerographic (dry toner) printers and presses typically employ uncharged substrates prepared from epoxy or acrylic resins, which are coated with charged polymeric coatings.
The highly cationic nature and low thermal stability of conventional digital offset receiving sheets render these sheets unsuitable for use in xerographic (dry toner) printers and presses utilizing heat and pressure fusing.
Moreover, receiver sheets for xerographic (dry toner) printers and presses exhibit low adhesion to liquid toners due in part to the absence of charge in the substrates used in these receiver sheets, thereby rendering them unsuitable for use in digital offset printing presses.
It is an object of the present invention to address these limitations in prior art receiver sheets by providing a receiver sheet that is suitable for use in both digital offset printing presses and xerographic (dry toner) printers and presses.